Magnesium alloys are promising lightweight structural materials; however, their high chemical activity makes them highly susceptible to corrosion. Micro-arc oxidation (MAO) is an effective surface treatment approach because it can produce a hard ceramic coating with strong adhesion and improved corrosion resistance. However, the micropores and microcracks inherently formed during the MAO process still limit its long-term protective performance. Therefore, further defect sealing and interfacial optimization are required. In this work, superhydrophobic MAO/MgAl-LDH-SDS composite coatings with enhanced corrosion resistance and self-healing ability were fabricated on an AZ31 magnesium alloy surface by combining MAO treatment with a two-step hydrothermal process. An in situ grown MAO/MgAl-LDH coating was first prepared, and the LDH layer was subsequently modified with sodium dodecyl sulfate (SDS) through intercalation/adsorption treatment. The optimum SDS modification conditions were identified as a concentration of 0.10 M and an immersion time of 8 h. The morphology, composition, and structure of the coatings were characterized by SEM, EDS, XPS, ATR-FTIR, and XRD. Their wettability, long-term corrosion resistance, and self-healing behavior were evaluated by water contact angle measurements, UV-vis analysis, self-cleaning tests, potentiodynamic polarization, long-term electrochemical impedance spectroscopy (EIS), hydrogen evolution tests, salt spray tests, and post-scratch EIS measurements. The results showed that the MAO coating was mainly composed of MgO and Mg2SiO4, while the in situ grown MgAl-LDH layer effectively sealed the pores and cracks of the MAO coating. After SDS modification, a low-surface-energy superhydrophobic interface was successfully established. The water contact angle of the MAO/MgAl-LDH-SDS coatings reached 160.22°, indicating excellent superhydrophobicity, self-cleaning ability, and air-retaining behaviors. Electrochemical measurements demonstrated that the MAO/MgAl-LDH-SDS composite coating exhibited markedly enhanced corrosion resistance compared with the MAO coating, with the corrosion current density decreasing from 1.687×10-8 A/cm2 to 2.923×10-10 A/cm2 and the low-frequency impedance modulus |Z|f=0.01 HZ increasing to 5.187×107 Ω·cm2. Long-term immersion, hydrogen evolution, and salt spray tests further confirmed its superior long-term protective performance. After 28 d of immersion, the hydrogen evolution volume decreased from 0.360 mL/cm2 to 0.031 mL/cm2, and the coating still maintained stable and durable corrosion protection under prolonged salt spray exposure. Overall, the corrosion protection performance followed the order of MAO<MAO/MgAl-LDH<MAO/MgAl-LDH-SDS. Post-scratch EIS results further demonstrated that modified coatings possessed self-healing capability, among which the SDS- modified coating showed better long-term healing stability. The enhanced corrosion protection of the MAO/MgAl-LDH-SDS coating was attributed to the synergistic effects of the MAO barrier layer, the LDH sealing layer, and the SDS- modified superhydrophobic interfacial layer. Specifically, the MAO layer provided the initial physical barrier, the LDH layer sealed surface defects and prolonged the diffusion path of corrosive species, and the SDS- modified layer lowered the surface free energy and created a stable air barrier that inhibited the penetration of water and Cl-. Meanwhile, the anion-exchange capability of the LDH layer and the sustained protection provided by intercalated or adsorbed SDS species further contributed to the active corrosion protection and self-healing behavior. As a result, the SDS- modified coating exhibited superior long-term corrosion resistance and enhanced long-term self-healing stability. These findings provide useful guidance for the design of high-performance MAO/LDH-based protective coatings for magnesium alloys.
Key words
magnesium alloy /
micro-arc oxidation /
layered double hydroxides /
superhydrophobic coating /
corrosion resistance
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